Free weights are best, but they have a drawback: Some parts of a lift are easier than others, so your muscles aren't being worked consistently. That's why Lautner often attaches giant rubber bands to a bar or dumbbell he's going to lift, and then anchors the bands to the base of a power rack or a pair of heavy dumbbells. "The bands create more tension, making the lift harder and forcing your muscles to peak out at the top of the movement," Yuam says. As a result, your body recruits more muscle fibers and works them harder, accelerating growth. Bands are available in most gyms.
Source: Men's Health